The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
The invention relates generally to hard, tough lightweight composite materials, and more particularly to B.sub.4 C/TiB.sub.2 composites and methods of making same.
U.S. Pat. No. 4,605,440 issued Aug. 12, 1986 to Halverson et al describes B.sub.4 C-reactive metal composites, particularly B.sub.4 C-Al composites, and methods of making same. The process involves achieving the conditions for liquid phase sintering of the metal and B.sub.4 C to occur. A variety of consolidation techniques can be used, with lower temperature and pressure methods being preferred. Fully dense composites with tailorable microstructures can be produced.
U.S. Pat. No. 4,718,941 issued Jan. 12, 1988 to Halverson et al. describes an improved infiltration process in which a chemically pretreated porous B.sub.4 C or other boron or boride ceramic matrix or sponge is infiltrated with molten aluminum or other metal to form metal-ceramic composites.
Previous attempts to fabricate B.sub.4 C-Ti composites using liquid-phase sintering or infiltration approaches were unsuccessful because of the rapid diffusion of boron and carbon atoms into titanium. This rapid diffusion phenomenon, "capillary-kinetic slowdown," results in the inhibition of the capillary action of molten titanium in porous B.sub.4 C compacts because of the formation of titanium borides and titanium carbides at the titanium surface prior to melting.
Attempts at conventional sintering and hot pressing at temperatures greater than 2273 K have always resulted in microstructures that were rich in TiB.sub.2, with B.sub.4 C as the minor accompanying phase.
Combustion synthesis of powder compacts has been used to produce a variety of refractory ceramic materials including nitrides and nitride-oxide composites. The process uses heat evolved during spontaneous chemical reactions between mixtures of solids or solids and gases produced as a combustion wave initiated by an ignition source rapidly propagates through the compact. The key to self-propagating high temperature synthesis (SHS) is that once initiated highly exothermic reactions will become self-sustaining and will propagate through the reactant mixture in the form of a combustion wave. As the combustion wave (front) advances, the reactants are converted to products. A major advantage of SHS as a process for the synthesis of materials is the energy savings associated with the use of self-sustaining reactions. However, the combustion synthesis of B.sub.4 C is not possible because it is a low exothermic material so the reaction does not generate enough heat to sustain the process.